Large quantities of methane, the main component of natural gas, are available in many areas of the world. However, a significant portion of that natural gas is situated in areas that are geographically remote from population and industrial centers (“stranded gas”). The costs of compression, transportation, and storage often makes the stranded gas' use economically unattractive. Consequently, the stranded natural gas is often flared. Flaring not only wastes the energy content and any possible economic value the natural gas may have but also creates environmental concerns.
To improve the economics of natural gas transportation and utilization, much research has focused on using the methane component of natural gas as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reacted to produce carbon monoxide and hydrogen (i.e., synthesis gas or “syngas”). In a second step, the syngas is converted to higher hydrocarbon products by processes such as Fischer-Tropsch synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the syngas. In addition, syngas may be used for the manufacture of ammonia, hydrogen, methanol, and other chemicals. Less traditional uses of syngas continue to be developed and have increased in importance in recent years, such as in the production of acetic acid and acetic anhydride manufacture. Among the promising new developments in syngas chemistry are routes to ethylene.
The syngas routes may be attractive in themselves, regardless of raw materials used; they may also provide the option to use alternative and ultimately cheaper raw materials such as coal and, in certain circumstances, natural gas. One of the attractions of syngas is that it can be manufactured from almost any raw material containing carbon; hence, the availability of feedstock is ensured.
The cost of syngas can be highly variable, depending on the effluent hydrogen/carbon monoxide ratio desired, the raw materials available, the production process, the scale of operation and extent of integration with other processes, and other factors. As described below, the current methods for producing syngas all have negative aspects, which result in inefficiencies, and in turn, a higher cost of producing syngas.
There are currently three primary reactions for converting methane to syngas. Those methods include: steam reforming (the most widespread), dry reforming (also called CO2 reforming), and partial oxidation. Steam reforming, dry reforming, and partial oxidation proceed according to the following reactions respectively:CH4+H2O+heat→CO+3H2  (1)CH4+CO2+heat→2CO+2H2  (2)CH4+½O2→CO+2H2+heat  (3)For a general discussion of steam reforming, dry (or CO2) reforming, and partial oxidation, please refer to HAROLD GUNARDSON, Industrial Gases in Petrochemical Processing 41-80 (1998), the contents of which are incorporated herein by reference for all purposes.
As noted in reaction 1, steam reforming is endothermic (requires heat); therefore, heat must be supplied to drive the reaction. One way to provide the necessary heat is to burn a portion of the available natural gas in process heaters. However, because some of the available natural gas is burned to heat the reactor, less natural gas is available to be converted to synthesis gas and the overall yield is lower than if all of the natural gas were converted to syngas. Other methods of supplying heat to the steam reforming reaction at remote well sites are often cost prohibitive. In addition, the steam reforming reaction is relatively slow, thereby requiring relatively long reactor residence times and correspondingly large reactors. These typically large steam reforming plants are usually not practical to set up at remote natural gas well sites.
Partial oxidation of hydrocarbons can also be used to produce syngas. Partial oxidation of hydrocarbons to produce syngas typically takes place in the presence of a catalyst. In catalytic partial oxidation (“CPOX”), natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The methane reacts exothermically with oxygen to form syngas. A specific example of a CPOX process is set forth in U.S. Pat. No. 5,510,056 to Jacobs, et al., incorporated herein by reference for all purposes.
Recently, CPOX of methane has attracted much attention due to its inherent advantages, such as the fact that due to the significant heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction. This is in contrast to steam reforming processes, which generally use external gas firing that decreases total liquid product yields (discussed above). CPOX also has space saving advantages. CPOX is a very fast reaction; therefore, reactor residence times are much less than those needed for steam reforming and thus, smaller reactors are acceptable. In addition, CPOX produces syngas with the optimal 2:1 H2:CO molar ratio for Fischer-Tropsch reactions, and has a simplified catalytic reaction plant section.
CPOX is not without its drawbacks. In CPOX, oxygen and methane must be mixed in the presence of a catalyst. Mixing of these components in certain temperature and pressure regimes can potentially lead to explosions, fires, and equipment failures. Because of this, CPOX has so far been substantially limited to low pressures due to the safety concerns. In addition, although it is possible to conduct a partial oxidation reaction in the presence of air or oxygen-enriched air it is often preferable to conduct the reaction in the presence of substantially pure oxygen because if other than substantially pure oxygen is used, diluants in the air (e.g., N2) will require the use of a much larger reactor, thus increasing the cost to build and operate and reducing or eliminating the size advantage of CPOX over steam reforming. Unfortunately, separation, compression, and handling of the substantially pure oxygen can be very expensive.
Another process for producing syngas is autothermal reforming (“ATR”). ATR is basically a combination of partial oxidation and steam reforming carried out in a single reactor. The heat released by the exothermic partial oxidation reaction is used to drive an endothermic steam reforming reaction in another part of the reactor.
One of the features of ATR is that it requires no external fuel. ATR also reduces, but does not eliminate, some of the safety issues involved with CPOX because a burner is used. The burner allows for the safe mixing and combustion of methane with oxygen. However, ATR also has negative aspects. For example, large amounts of CO2 are generated in the partial oxidation portion of an ATR reactor. This reduces the overall conversion of methane to CO. Additionally, removal of that CO2 increases the expense of the overall processing scheme. A detailed discussion of ATR is included on pages 61-66 of the GUNARDSON referenced cited above.
With regard to the membrane art, research done by Prabhu, Radhakrishnan, and Oyama (PRABHU, ET AL., Supported Nickel Catalysts for Carbon Dioxide Reforming of Methane in Plug Flow and Membrane Reactors, APPLIED CATALYSIS A: GENERAL 241-52 (1999) (“PRABHU, ET AL.”)), incorporated herein by reference in its entirety for all purposes, discloses the use of a hydrogen permeable membrane to separate hydrogen from the reaction product of a dry reforming reaction to shift equilibrium conditions and increase the methane conversion in the reactor. As is shown in FIG. 9 of PRABHU, ET AL., the Vycor® membrane used was effective up to a temperature of at least 1023 K. It should be noted that the PRABHU, ET AL. reference does not teach the combustion of the permeated hydrogen and instead uses a Hoskins tubular furnace to drive the endothermic dry reforming reaction. Likewise, U.S. Pat. No. 5,637,259 to Galuszka et al., incorporated herein by reference for all purposes, discloses the use of a hydrogen permeable membrane to separate hydrogen from the reaction product of a dry reforming reaction and a catalytic partial oxidation reaction to shift equilibrium conditions and increase the methane conversion and the H2 and CO selectivities in the reactor. Like PRABHU ET AL., Galuszka et al. does not teach the combustion of the separated hydrogen to drive the reaction or the use of a membrane in conjunction with a stream reforming reaction.
Because syngas is used in both methanol, Fischer-Tropsch, and other syntheses, the demand for syngas remains high. This has fueled syngas research, which has resulted in processes such as steam reforming, CPOX, and ATR. However, while these competing processes have benefits, they also have flaws or limitations, which ultimately limit their utility. Therefore, there exists a need for new processes that exhibit at least some of the positive features of these competing processes, while reducing or eliminating the negative features or limitations.